1. Field of the Invention
The present invention relates generally to the field of capacitors, and more particularly to embedded multi-layer capacitors formed in a low-temperature co-fired ceramic (LTCC) substrate.
2. Description of the Related Art
Conventionally, electronic circuit components, including silicon chips, have been mounted on printed circuit boards. More recently, in order to reduce the size associated with conventional printed circuit boards, low-temperature co-fired ceramic (LTCC) substrates have been used.
A typical LTCC configuration comprises multiple layers of ceramic "tape" which are used to provide the base structure upon which to form various electronic components and electrical connections. The tape is formed from a powdered ceramic, mixed with a binder. For example, one type of ceramic tape available from Dupont is known as "Green Tape 951." The electronic components that can be formed include resistors, capacitors, inductors and the like. The electrical connections, formed through each tape layer are known as "vias." The components are formed by punching holes in the tape as appropriate, and layering on metal, dielectrics, insulators, etc. Several layers of tape may be used in order to form the desired circuitry. The tape layers are then pressed together and fired in an oven to remove the binder and to sinter the ceramic powder. Components which are too large or too difficult to form within the ceramic tape layers, such as silicon chips, may be surface mounted on the hardened substrate. The resulting substrate is usually less than 1".times.1" thus providing a compact circuit package.
FIG. 1 shows an example of the structure of a basic capacitor 10 formed within an LTCC substrate. A standard process to form this structure will now be described, with reference to FIG. 1. First, a hole is punched in a first tape layer 12 to form an opening. A silver, silver palladium, or similar paste is wiped across the hole to form a via 16, which is used as one terminal connection for the capacitor 10. Next, a first electrode 18 may be formed on top of the via 16 using silver, silver palladium or other similar electrode paste. After the electrode 18 has dried, a dielectric 20 is formed on top of the electrode 18. A dielectric paste is usually used, which when hardened, provides the desired dielectric properties. A second electrode 22 is then formed on top of the dielectric layer 20. These various component layers are commonly formed using a screen printing process. A second ceramic tape layer 14 having a via 24 is then pressed on top of the first layer 12. The second via 24 provides a second terminal for the capacitor 10. After the substrate is fired at 750-950.degree. C., a capacitor structure 30 is formed as shown in FIG. 2. A top view of the capacitor structure 30 is shown in FIG. 3.
This procedure forms what is known in the art as a single layer ceramic capacitor (SLCC), also known as a mono-layer capacitor. Another embodiment of an SLCC is shown in FIG. 4. Specifically, the vias 42, 44 may be formed on the side of the electrodes 46,48, or with one via 42 in the middle of one electrode 46, and one via 44 on the side of the other electrode 48. The electrodes and dielectrics may also be formed as circles, squares or rectangles as shown in FIGS. 5(A)-5(C).
For standard capacitor configurations, the capacitance of a structure is determined according to the following formula: C=(kA)/t, where k is the dielectric constant of the dielectric material, A is the surface area of the electrodes, and t is the thickness of the dielectric, as shown in FIG. 7. Note that in FIG. 7, the dielectric area A and the electrode area A are presumed to be the same, but in practice the dielectric is usually made larger to ensure that the electrode layers do not touch. Thus, by changing the dielectric material, the capacitance value may be changed. FIG. 6 illustrates an SLCC in which a different dielectric is used, in order to change the capacitance. Instead of using a dielectric paste, the dielectric may be formed from a high temperature ceramic tape (1100-1400.degree. C.). The high temperature tape 66 is fired separately, and then placed on the first electrode 72. The dielectric constant of the high temperature tape 66 is several orders of magnitude greater (k=20-20,000) than the dielectric constant of the standard tape (k=7-8) used to form the layers 62, 64. Standard tape 68,70 may be used around the dielectric to provide a constant thickness between the main layers 62, 64.
Two articles which discuss LTCC technology include "Characterization and Performance Prediction for Integral Capacitors in Low Temperature Co-Fired Ceramic Technology," Delaney et al., IEEE Transactions on Advanced Packaging, Vol. 22, No. 1, February 1999, pgs. 68-77; and "Characteristics of the Electrical Performance of Buried Capacitors and Resistors in Low Temperature Co-Fired (LTCC) Ceramic," Delaney et al., 1998 Electronic Components and Technology Conference, pgs. 900-908, the disclosures of which are herein incorporated by reference. While these articles seek to address the problem of providing capacitors with increased capacitance, the capacitors are still confined to being formed within a single layer of ceramic tape. The disclosed processes cannot make high capacitance capacitors and they require numerous types of dielectric materials in order to create different capacitances.
Since there is a practical limit to the dielectric constant that can be achieved, single layer capacitors do not provide sufficient capacitance within a reasonable area, for many applications. Thus, for high value capacitances, external capacitors are often surface mounted on the ceramic substrate. An example of one type of capacitor used for this purpose is shown in FIG. 8. Multiple layers of electrodes are formed in a discrete ceramic capacitor, and are used in order to increase the capacitance, while still providing a relatively small component. Adding external components, however, increases the costs associated with the LTCC circuit.
It would thus be desirable to have a multi-layer capacitor, embedded in the ceramic block, to reduce costs associated with manufacturing and attaching external capacitors, and to provide increased capacitance as compared to SLCCs.